Isotopic analysis of materials provides increased amount of information relative to information generated by traditional chemical analyses. Although qualitative and quantitative structural analyses identify the chemical composition of a compound or individual molecules of the compound, isotopic analysis provides additional information regarding the source, origin and formation of such compounds and molecules.
Mass spectrometers are well known and are used for wide ranging applications, such as isotope ratio monitoring, chemical analysis ranging from environmental analysis (e.g., detection of poisons) to the analysis of petroleum products, tracing of metals and biological materials. Mass spectrometers produce charged particles (e.g., ions) from chemical substances that are to be analyzed. After producing the ions, the mass spectrometers use electric and magnetic fields to measure the mass of the ions for isotope ratio monitoring.
Mass spectrometers are generally described in U.S. Pat. No. 4,638,160 to Soldzian et al. and U.S. Pat. No. 5,194,732 to Bateman, both of which are incorporated herein by reference. Mass spectrometers manufactured by Cameca are disclosed at www.cameca.fr, mass spectrometers manufactured by GV Instruments are disclosed at www.gvinstruments.co.uk, and mass spectrometers manufactured by Thermo Electron Co. are disclosed at www.thermo.com.
Design and construction of a mass spectrometer with high sensitivity to measure isotope ratios require compromises in design and construction. High absolute sensitivity and high abundance sensitivity are required to make isotope ratio measurements of elements with wide (e.g., 108) isotope ratios. In order to make such measurements with an extremely small sample, it is necessary to simultaneously measure the isotopes.
For example, a wide dynamic range is required to determine weapon yield using ratios of 242Pu and 244Pu to 239Pu, and tailing from the major peak at 239 onto the small peaks must be limited (high abundance sensitivity) in order to make a meaningful measurement.
Samples having smaller sizes may produce signals with meaningful intensities for only a short period of time (e.g., minute or less). Signal intensity typically changes rapidly under such circumstances. Scanning mass spectrometers that can only measure one isotope at a time are at a disadvantage under these circumstances, since the signals from the isotopes of interest may have to be interpolated to obtain isotope ratios.
Prior mass spectrometers manufactured by such entities as Thermo Finnegan and GV Instruments use arrays of Faraday cups and are configured with miniaturized channeltron multipliers for pulse counting. Such channeltron multipliers have high background counts and no more than 70% efficiency. The high background counts tend to limit sensitivity. Mass spectrometers made by the above-noted entities do not have sufficient dispersion between adjacent isotopes to accommodate full-sized multipliers that have 100% efficiency and background levels of about 3 counts/minute.
Instruments used for isotope ratio measurements typically had a single magnetic sector. Such instruments operated in the scanning or peak stepping mode and were not practical to set up to collect an entire U or Pu spectrum simultaneously.
FIG. 1 shows a schematic of a prior art mass spectrometer 100 designed to measure the isotopic composition of a sample. The mass spectrometer 100 includes an ion source 102 configured to generate a beam of ions 104 that are characteristic of the various element(s) present in the sample whose isotopic composition is to be determined. The beam of ions 104 is received in a magnetic sector 106 which disperses such beams of ions into separate beams 108–110 of discrete mass-to-charge ratios. Beams 108–110 are respectively received by detectors 112–114, which are typically Faraday cup collectors. The isotopic composition of the element in question is determined by simultaneous measurement of signals generated by detectors 112–114. In the arrangement of FIG. 1, the mass dispersion of beams 108–110 is solely due to the magnetic sector 106.
FIG. 2 shows a schematic of another prior art mass spectrometer 200 having an ion source 202 that generates a beam of ions 204 that are dispersed by a magnetic sector 206 into a plurality of beams 207, 208 according to their mass-to-charge ratios. Beams 207, 208 enter an electrostatic analyzer 210 which cooperates with the magnetic sector 206 to produce an image on detector 212, the image being focused both in velocity and direction. The mass spectrometer 200 includes a detector 216 for detecting different isotopes of a sample. The electrostatic analyzer 210 is used for double focusing to be maintained over a wide range of deflection angles and focal lengths of the electrostatic analyzer 210. The dispersion of beams 209, 211 exiting the electrostatic analyzer 210 is solely due to the magnetic sector 206. The electrostatic analyzer 210 is used to provide energy focusing of the ion beams in order to filter out ions that have scattered off of internal walls of the mass spectrometer vacuum housing or ions that have scattered due to collisions with residual gas in the vacuum system.
Prior approaches necessitate use of miniaturized detectors that are less than 100% efficient and have a high background noise level. Individual ion beams cannot readily be separated far enough apart to allow use of full sized Faraday cups or discrete dynode pulse counting detectors for each separated beam with existing approaches.
FIG. 2a shows a schematic of a prior art commercial isotope ratio mass spectrometer having an ion source 202 that generates a beam of ions 204 that are dispersed by a magnetic sector 206 into a plurality of beams B according to their mass to charge ratios. Beams B are simultaneously focused by the magnetic sector 206 into multiple miniature faraday cup collectors C, with one of the beams being focused into a miniature electron multiplier C1.